Ultra-high molecular weight ethylene polymers are particularly suited for the production of materials that need high mechanical properties, such as the impact and abrasion resistance required by parts destined for gear or arthroprosthesis, and the high tensile strength and modulus required by highly oriented fibers such as sailcloth.
It is known that it is particularly desirable that the ultra-high molecular weight polymers of ethylene and .alpha.-olefins (with intrinsic viscosity values in tetrahydronaphthalene at 135.degree. C. not lower than 8 dl/g, generally from 8 to 30 dl/g, i.e., in terms of molecular weight, from 1,000,000 to 7,000,000) be in the form of very fine powders in order for them to offer good flowability and compactness. In fact, due to the high viscosity that the ultra-high molecular weight polymers present in the molten state even at high temperatures, the normal molding processes that use melted polymer are not adequate for the manufacture of products. In such cases an alternative to molding is provided by some techniques for converting powders in compact semi-finished materials, which can be used to manufacture finished products with the use of the proper machines, such as lathes and cutters. Said techniques are:
I) "compression molding", consisting of obtaining thick sheets (up to 80 mm thick) by way of cohesion and compaction of polymer powders at high temperatures and under high pressure;
II) "ram extrusion" consisting of obtaining cylindrical rods with a diameter up to 80-100 mm using ram extruders.
The finer the particles of the starting polymer powders, with a regular form and a controlled particle size distribution, the better the physico-mechanical properties of the above mentioned semi-finished materials. An additional factor that improves the workability and quality of the finished product is the porosity of the polymer particles.
The above mentioned characteristics of the powders cannot be obtained optimally by way of milling the solid catalyst component or the polymers produced thereby, because milling produces powders with unequal dimensions, irregular form, and uncontrollable particle size distribution. When the solid catalyst component is milled, all irregularities are reproduced in the resulting polymer due to the replication phenomenon. Another undesired consequence of milling is decreased consistency and porosity of the polymer particles.
The morphologic irregularity of the polymer powders obtained by milling causes poor flowability, and a deterioration of the physico-mechanical properties of the products obtained by compression molding.
Therefore, it is desirable to have catalyst components in the form of very fine particles having a regular morphology and controlled particle size distribution, which could produce, by way of replication, polymer powders with good shape characteristics and flowability, and suited to be used in compression molding and ram extrusion processes.
Catalyst systems capable of producing high molecular weight polymers are already known in the art.
U.S. Pat. No. 1,513,480 describes catalyst components for the polymerization of olefins obtained by way of reaction between a transition metal compound, selected from halogenated compounds and organic compounds containing oxygen, and a solid product, insoluble in hydrocarbons, comprising compounds containing at least one M metal and a halogen, obtained by decomposition, to metal dihalide or products containing metal dihalides, of a complex of general formula
MX.sub.2 .multidot.nAlRX'.sub.2 .multidot.pAlX.sub.3
where M is magnesium or manganese; X' is a halogen; X is a halogen or an alkoxy group; R is a hydrocarbon radical; n is a number from 1 to 4, p is less than or equal to 1, and n+p ranges from 1 to 4, extremes included.
The catalyst components, as will be demonstrated in the comparative example, in the form of fine particles, prepared as described in the above mentioned patent do not seem to be capable of producing polymer powders suitable for compression molding.
Published European Patent Application No. 317,200 describes a process for the preparation of ultra-high molecular weight polyethylene by using a catalyst comprising a solid catalyst component obtained by way of reaction between: a) the product of the reaction between a magnesium dihalide and a titanium tetraalcoholate, and b) the product of the reaction between an aluminum trihalide and silicon tetraalcoholate.
The use of a catalyst obtained from the catalyst component described in said patent application allows one to obtain a polymer in particle form with variable average dimensions, between 195 and 245 in the examples, and controlled particle size distribution, particularly adequate for gel spinning processes.